Research On An Islanding Detection Method For Droop Control Grid-connected Inverters Based On Impedance Identification

The droop control grid-connected inverters have been widely used in new energy generation system because of plug and play,power sharing without communication and adaptability to weak grid.The droop control grid-connected inverters can work in islanding mode,but the lack of islanding detection function will bring huge security risks to grid-connected system when grid restored.At present,islanding detection algorithms are mostly proposed for direct current control grid-connected inverters,which are not suitable for droop control grid connected inverters.In order to solve the above problems,this thesis analyzes the adaptability of the current islanding detection algorithm to the droop control grid connected inverters.In addition,this thesis proposes an islanding detection method for the droop control grid connected inverters based on impedance identification,which realizes fast detection and zero NDZ,and improves the safety of the droop control grid-connected inverters.The main research work and innovation are as follows:(1)In view of the adaptability of the current islanding detection algorithm to the droop control grid-connected inverters,the adaptability of the passive islanding detection algorithms and the active islanding detection algorithms to the droop control grid connected inverters are analyzed respectively.It is found that the current islanding detection algorithms are not suitable for the droop control grid-connected inverters.Therefore,an islanding detection method for the droop control grid connected inverters based on impedance identification is proposed.(2)In order to solve the problem that impedance identification is not suitable for grid with background harmonics,this thesis analyzes the influence of background harmonics on impedance identification.Moreover,a disturbance injection method based on predetection is proposed,which improves the effectiveness of disturbance signal injection and reduces the impact of disturbance signal on grid power quality.(3)In view of the complexity of impedance identification calculation and large calculation space,this thesis analyzes the principle and existing problems of extracting harmonics by DFT and FFT,and proposes an impedance identification method based on RDFT,which greatly shortens the impedance identification time on the premise of ensuring the identification accuracy.(4)In order to solve the problem that power supply restoration after islanding brings security risks to droop control grid-connected inverters,this thesis analyzes the mechanism of security risks,and a pre-synchronization strategy based on amplitude closed-loop and phase closed-loop is proposed to ensure the security of grid connected system after grid restoration.